Time that an object is in the air

In a question, it asks for the time that an object is in the air; at first I thought it meant the time at max height, but I was told it should be the time at max height divided by 2 because the object goes up and down. But if the object goes up and down, wouldn't you mult by 2?

Here's the question:
An object fired straight upward with an initial velocity of 1.0 x 10^2 m/s is subjected to the effects of gravitation.
In a previous question, I solved for max height, which was 5.1 x 10^2 m. (I'm not sure if this is related to the next question though).
The time the object is in the air is
a. 4.5 s
b. 9.0 s
c. 20 s
d. 10 s